Transcriptional regulation of the human IgE receptor (Fc epsilon RII/CD23) by EBV. Identification of EBV-responsive regulatory elements in intron 1.

Abstract

EBV infection of B cells induces the B cell activation Ag, CD23 (Fc epsilon RII). CD23 remains constitutively expressed at high levels in all EBV-immortalized B cells and likely plays an important role in the initiation and maintenance of immortalization by EBV. By utilizing an EBV-negative Burkitt's lymphoma line (BJAB) and EBV-positive sublines derived from it by in vitro infection, we have examined the molecular mechanisms involved in the regulation of CD23 by EBV. By nuclear runoff analysis, we have found that induction of CD23 is mediated by transcriptional activation that occurs in the presence of the transformation-competent B958 virus but not in the presence of the nontransforming P3HR-1 strain of EBV. To identify EBV-responsive transcriptional regulatory elements of CD23, we have performed reporter gene assays using plasmids containing fragments of the CD23 gene derived from its 5' terminus and adjacent flanking region transfected into EBV-positive and -negative BJAB lines. We have identified a 534-bp fragment of the gene which enhances transcription from a heterologous promoter (SV40) and reporter gene (chloramphenicol acetyltransferase) only in the presence of transformation-competent strains of EBV. Deletion of 144 bp of intron 1 from the 3' end of this fragment results in loss of EBV-responsive enhancer activity. The finding of an EBV-responsive enhancer element of CD23 is supported by mobility shift assays that demonstrated the formation of specific DNA-protein complexes between nuclear protein from transforming EBV-positive cells and the 144-bp intron sequence. These studies suggest that the transcriptional activation of CD23 by transforming strains of EBV involves regulatory elements that are located within the first intron of the gene.